Method for analyzing the game of a user of a racket

ABSTRACT

A method for analyzing the game of a user of a racket (Ra) includes detecting an impact from representative measurements of a shock to the racket (Ra) provided by a sensor assembly. The sensor assembly includes at least one sensor sensitive to shocks and is connected to the racket (Ra). 
     A moment of impact is associated with a detected impact, using the measurements transmitted by the sensor assembly. 
     Impacts that are not related to strokes from a set of pre-determined strokes are eliminated on the basis of angular rotational velocity measurements, provided by a gyrometer (G) of the sensor assembly with at least one measurement axis and linked to the racket, taken during an interval of time around the moment of impact.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of International Application No.PCT/EP2013/058719, filed on Apr. 26, 2013, which claims the benefit ofFrench Application No. 1254257, filed May 10, 2012, and FrenchApplication No. 1259662, filed Oct. 10, 2012. The contents of all ofthese applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention relate to a method for analyzing the gameof a user of a racket, wherein an impact, notably of an associatedprojectile, is detected on the racket.

2. Description of the Related Art

The term projectile is understood to refer to a ball, in general, ingames or sports such as tennis, table tennis, squash, racquetball, or,for example, a shuttlecock for badminton.

Systems are known comprising rackets equipped with sensors to providedata related to the player.

United States patent publication no. 2011/183787 relates to a racketequipped with at least one sensor, for example an accelerometer, ananemometer, a pressure sensor, a stress sensor or a piezoelectricsensor.

This document is very general and concerns a racket equipped with atleast one sensor, and a device incorporated into the racket, controlledby a motor, to modify at least one feature of the racket, for examplestiffness or string tension, as a function of the sensor signal. Such adevice seems to have a high price and weight.

U.S. Pat. No. 5,757,266 relates to an electronic system designed tomonitor the capacity of a player to correctly center his ball on thestringbed of his racket on which a plurality of sensors, distributedaround the periphery of the stringbed or strings of the racket, make itpossible to detect relative arrival times of the waves created by animpact of a ball on the racket. Based on these signals, the position ofthe impact of the ball on the stringbed can be computed. Thiscomputation is carried out by a microprocessor embedded in the racket.Furthermore, a display device is provided on the racket to provide theplayer with information relating to the centering of the ball.

Such a system is not suitable for determining impacts on the racket thatare not ball impacts. Furthermore, the number of sensors is large andthe cost is high.

United States patent publication no. 2005/0239583 relates to a strikingor percussive device, such as a racket, a bat or a baton, equipped withsensors including an acceleration sensor, to determine the velocity ofthe displaced object or of the striking device.

The described system seems relatively limited to evaluating a velocityof a striking or percussive element, or a velocity of the displacedobject.

U.S. Pat. No. 6,134,965 relates to a racket equipped with vibrationsensors, the frequency of which is analyzed to determine the velocity ofthe struck ball.

The described system seems relatively limited to evaluating a velocityof a striking or percussive element, or a velocity of the ball.

SUMMARY OF THE INVENTION

One aim of embodiments of the invention is to propose an improved methodfor analyzing the game of a user of a racket and of an associatedprojectile, making it possible to analyze the player's game in real ordelayed time.

Another aim of embodiments of the invention is to improve the precisionof the detection of an impact of the projectile on the racket.

Thus, according to an aspect of embodiments of the invention, a methodis proposed for analyzing the game of a user of a racket, wherein:

-   -   an impact on the racket is detected from representative        measurements of a shock to the racket provided by a sensor        assembly comprising at least one sensor sensitive to shocks        linked to the racket in a fixed manner in terms of movement;    -   a moment of impact is associated with a detected impact, from        the measurements transmitted by the sensor assembly; and    -   the impacts that are not related to strokes from a set of        pre-determined strokes are eliminated on the basis of angular        rotational velocity measurements, provided by a gyrometer of the        sensor assembly with at least one measurement axis and linked to        the racket in a fixed manner in terms of movement, taken during        an interval of time around said moment of impact.

The game can be tennis, squash, table tennis, badminton, or any racketsport.

Such a method makes it possible to improve analysis of the game of auser of a racket and of an associated projectile, making it possible toanalyze the game of the player in real or delayed time.

Such a method also makes it possible to eliminate impacts that are notpart of the game in order to analyze only the strokes of the game.

In a method of implementation, said sensor assembly comprising at leastone vibration sensor, an impact on the racket is detected frommeasurements of vibrations transmitted by the vibration sensor, bycomparison of a parameter representing the vibrations with a threshold.

A vibration sensor, for example a piezoelectric sensor, makes itpossible to detect easily and at low cost.

According to one method of implementation, said sensor assemblycomprising an accelerometer with at least one measurement axis, animpact on the racket is detected when a parameter depending on thevariations over time of the axial linear accelerations and/or variationsover time of the angular rotational velocities is above a threshold.

The use of such sensors makes it possible to analyze the movements ofthe strokes, even if they saturate the sensors at times.

In one method of implementation, impacts that are not related to strokesfrom a set of predetermined strokes are eliminated, on the basis of acomparison between a value representing the angular rotational velocityalong an axis during an interval of time around the moment of impact anda threshold.

Thus, only strokes that are part of the game are analyzed.

According to one method of implementation, said gyrometer comprises atleast two measurement axes, impacts that are not related to strokes froma set of predetermined strokes are eliminated, on the basis of acomparison between a first value representing the angular rotationalvelocity along a first axis during an interval of time around the momentof impact and a second value representing the angular rotationalvelocity along a second axis during said interval of time.

Thus, false impact detections are avoided, notably lateral impacts, forexample when the player hits the side of his racket against his sportsshoes to remove clay, or else when he hits the side of his racket in hishand.

In one method of implementation, said gyrometer comprises threemeasurement axes and said sensor assembly comprises at least oneaccelerometer with three measurement axes, the attitude of the racket isdetermined with respect to a terrestrial frame of reference from themeasurements of the axial linear accelerations and the measurements ofthe angular rotational velocities along said measurement axes.

The fact of determining, continuously, the attitude of the racket withrespect to a terrestrial frame of reference makes it possible to avoiddetections of strokes that one does not wish to detect.

According to one method of implementation, a stroke is classified amonga set of strokes by association of a stroke with a movement of theracket for which, around the moment of impact, the rotational velocityof the racket is essentially along a determined axis and the attitude ofthe racket is essentially at a determined attitude.

The terms orientation or attitude refer to the angular separations ofthe axes of the reference frame linked to the racket with respect to theterrestrial frame of reference axes. This attitude data item isconventionally expressed by a quaternion rotational matrix, Euler anglesor any other suitable representation. For determined strokes theattitude of the racket is preferably not random.

It is therefore easy to determine a stroke among a set of strokes, suchas a service, a forehand stroke or a backhand. The player or his coachcan thus evaluate the correct execution of the various strokes by theplayer, to improve his technique.

Indeed, for a movement to be a stroke, it is desirable to move theracket correctly and have the racket in a particular orientation. Forexample, in tennis, it is hard to serve with the racket horizontal.

Thus, the determination of the stroke and the evaluation of the correctexecution of a stroke are improved.

According to one method of implementation, a stroke is classified amonga set of strokes, furthermore on the basis of an item of informationrepresenting the left- or right-handedness of the player, and of thesign of the angular rotational velocity along the determined axis.

Precision is thus improved. The set of the strokes is larger and theinvention makes it possible to discern more strokes, with a higherprecision.

For example, the item of information representing the left- orright-handedness of the player is provided by the player or learntautomatically during the game, for example in particular phases of thegame.

If the player is learning during the game, the player does not need to“manually” enter whether he is left or right handed.

In one method of implementation, a stroke is detected among a set ofstrokes by associating a stroke with a determined form of a projectionsignal of a vector representing the attitude of the racket onto an axisdetermined over an interval of time around the moment of impact.

Thus, the player can pay attention to the correct execution of thevarious strokes, obtain statistics on his game, and improve histechnique.

According to one method of implementation, said gyrometer comprises atleast two measurement axes and the impact is due to a projectile; theintensity of an effect given to the projectile is determined, at themoment of impact, from a comparison of the angular rotational velocityalong a first axis during an interval of time around the moment ofimpact and of the angular rotational velocity along a second axis duringsaid interval of time.

Thus, the player obtains statistics on the use that he makes of theeffects, on their intensity and can learn and progress in the use ofeffects.

In one method of implementation, said axes comprise a first transverseaxis in the direction of the width of the racket and a second transverseaxis in the direction of the thickness of the racket, and a backspineffect is differentiated from a topspin effect on the basis of the signof the angular rotational velocity along the second transverse axis andan orientation of the racket during said interval of time.

It is then easy to tell the difference between a backspin effect and atopspin effect.

According to one method of implementation, the axes comprise alongitudinal axis oriented from the shaft toward the head of the racket,said gyrometer with at least one measurement axis is capable ofdelivering a rotational velocity along the longitudinal axis, and theimpact is due to a projectile striking a longitudinal impact strip onthe stringbed of the racket, wherein the impact of the projectile havingtaken place, is determined from a variation of the angular rotationalvelocity along the longitudinal axis over an interval of timeimmediately following the impact.

Thus, the player has access to a statistic on the centering of theprojectile on the stringbed of the racket, he can improve his impactposition when he strikes the projectile, and thus optimize hisregularity, his precision and his energy loss.

In one method of implementation, said gyrometer with at least onemeasurement axis is capable of delivering a rotational velocity along afirst transverse axis in the direction of the width of the racket, andsaid determination of the longitudinal impact strip is corrected by anitem of information representing the velocity of the racket around themoment of impact.

The determination of the longitudinal impact strip is thus improved.

According to one method of implementation, the sensor assembly comprisesat least one vibration sensor and the impact is due to a projectilestriking a radial impact strip, wherein the impact of the projectilehaving taken place, is determined on the basis of the energy and thephase of the signal transmitted by the vibration sensor due to theimpact.

Thus, the player has access to a statistic on the centering of theprojectile on the stringbed of the racket, he can improve the positionof impact of the projectile, and optimize his regularity, his precisionand limit his loss of energy.

Thus, by knowing at what velocity the ball is struck, it is possible totrain oneself to optimize the stroke, improve the velocity of thestrokes, strike harder, but without losing precision.

According to one method of implementation, the sensor assembly comprisesan accelerometer with three measurement axes and/or a gyrometer (G) withthree measurement axes, and the impact is due to a projectile, andduring a start of a phase of the game, a launch velocity of theprojectile is respectively computed from the measurements of the axialaccelerations and/or the measurements of the angular rotationalvelocities during the phase of acceleration of the racket preceding theimpact.

Thus, by knowing at what velocity the ball is struck, it is possible totrain oneself to optimize the stroke, improve one's ball velocity onthese opening phases of the game, strike harder, but without losingprecision.

In one method of implementation, the computed launch velocity of theprojectile is corrected on the basis of the knowledge of a zone ofimpact of the projectile and/or of the intensity of the effect given tothe projectile.

Thus, the precision of the computation of the velocity is improved.

According to one method of implementation, in addition, the location ofthe player on the game space is determined from the data provided by asystem for locating the player or the racket.

Thus, it is possible to obtain more statistics by recovering thedistribution of the strokes made by the player as a function of hisposition on the court. This information makes it possible to highlightbehaviors of the player, and to improve them.

In one method of implementation, the sensor assembly comprises at leastone accelerometer, and when during an interval of time around the momentof impact the signals of said sensor or sensors are saturated, anextrapolation of the signals provided by the sensor or sensors iscarried out over said saturation time interval.

Thus, even in the case of saturation of the sensors, the precisionremains excellent.

In one method of implementation, one provides, in real or delayed time,qualitative and/or quantitative statistics relative to the player'smanner of playing.

It is thus possible for the player or his coach to be able to track hislevel of play either directly, or in a delayed manner, to improve. Thelevel of play can be tracked in a qualitative and/or quantitative way.

Also proposed, according to another aspect of the invention, is a systemfor analyzing the game of a user of a racket, comprising:

-   -   means for detecting an impact on the racket from measurements        representing a shock experienced by the racket provided from a        sensor assembly comprising at least one sensor sensitive to        shocks linked in a fixed manner to the racket in terms of        movement;    -   means for associating a moment of impact with a detected impact,        on the basis of the measurements transmitted by the sensor        assembly;    -   a gyrometer, from the sensor assembly, with at least one        measurement axis and linked in a fixed manner to the racket in        terms of movement; and    -   means for eliminating the impacts not related to strokes from a        set of predetermined strokes on the basis of angular rotational        velocity measurements, provided by said gyrometer, taken during        an interval of time around said moment of impact.

For example, said sensor assembly is mounted in a fixed manner in anouter casing equipped with fixing means adapted to be mounted/dismountedat will on the racket, or is mounted in a fixed manner on the racket.

According to an embodiment, said sensor assembly is mounted on theracket in a fixed manner in such a way that two measurement axes of saidsensor assembly form an angle of 45° with a first transverse axis in thedirection of the width of the racket and a longitudinal axis in thedirection of the length of the racket.

In the case of the outer casing equipped with fixing means adapted forbeing mounted/dismounted as desired on the racket, the system comprisesan autonomous part which can be adapted to any racket.

In the case of the outer casing mounted in a fixed manner on the racket,the system comprises sensors mounted on the racket in a permanentmanner, which makes it possible to optimize the operation to thefeatures of the racket.

In one embodiment, the sensor assembly is mounted in a fixed manner onthe racket and comprises an accelerometer and a gyrometer arranged inthe shaft of the racket at the bottom of the grip, and a vibrationsensor arranged on the shaft of the racket between the grip and thebottom of the head of the racket.

This is a case of sensor placement giving improved results.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood after studying a few embodimentsdescribed by way of in no way limiting examples and illustrated by theappended drawings wherein:

FIG. 1 schematically illustrates a method for analyzing the game of auser of a racket and an associated projectile according to oneembodiment of the invention;

FIG. 2 schematically illustrates the axes of the racket corresponding tothe measurements, or to which one refers the measurements of the sensorslinked to the racket in a fixed manner in terms of movement, accordingto one embodiment of the invention;

FIG. 2 a schematically illustrates the measurement axes or the axes towhich one refers the measurements of the sensors linked to the racket ina fixed manner in terms of movement, inclined at 45° with respect to theracket axes, according to one embodiment of the invention;

FIG. 3 schematically illustrates the elimination of unwanted impacts,according to one embodiment of the invention;

FIG. 4 illustrates the determination of the attitude or orientation ofthe racket, according to one embodiment of the invention;

FIG. 5 schematically illustrates the detection of a stroke among a setof strokes, according to one embodiment of the invention;

FIG. 6 schematically illustrates the orientation of the longitudinalaxis of the racket with respect to the gravity vector, by projection ofthe longitudinal axis onto the gravity vector, according to oneembodiment of the invention;

FIG. 7 schematically illustrates the evolution of this projection duringa service, according to one embodiment of the invention;

FIG. 8 schematically illustrates the orientation of the first transverseaxis x in the direction of the width of the racket with respect to thegravity vector, by projection of the first transverse axis onto thegravity vector, according to one embodiment of the invention;

FIG. 9 schematically illustrates a table of the result of adetermination of a forehand or backhand stroke, according to oneembodiment of the invention;

FIG. 10 schematically illustrates three longitudinal impact strips, onehigh-impact strip, one medium-impact strip, and one low-impact strip, ofthe racket, according to one embodiment of the invention;

FIG. 11 schematically illustrates the variation of the angularrotational velocity along the longitudinal axis z oriented from theshaft toward the head of the racket during an impact of the projectile,according to one embodiment of the invention;

FIG. 12 schematically illustrates three radial impact strips of theracket, according to one embodiment of the invention;

FIG. 13 schematically illustrates the phase Φ₁ of the signal conveyed bythe piezoelectric sensor as a function of the normalized energy E_(E1),in the case of FIG. 12, according to one embodiment of the invention;

FIG. 14 schematically illustrates the evolution over time of the racketduring a service, according to one embodiment of the invention;

FIG. 15 schematically illustrates the evolution over time of theacceleration of the racket in a service according to one embodiment ofthe invention; and

FIG. 16 illustrates the computation of the velocity of the projectileaccording to one embodiment of the invention.

In all the figures, the elements having the same references are similar.The examples described relate to tennis, without however being limiting,because the invention is applicable to any type of game or sportrequiring the use of a racket and an associated projectile.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a method for analyzing the game of a user of a racketand of an associated projectile, wherein an impact of the projectile isdetected on the racket, with which a moment of impact t₀ is associatedfrom measurements representing a shock experienced by the racket, forexample by a sensor assembly comprising a gyrometer with at least onemeasurement axis. The sensor assembly may furthermore comprise at leastone vibration sensor, and/or at least one accelerometer with at leastone measurement axis. Once the moment of impact t₀ is known, the datasupplied by the sensors around the moment of impact t₀ is analyzed todetermine whether it is a stroke among the set of strokes, and todetermine other parameters of the game, such as the effect, thevelocity, etc.

As the example in FIG. 2 illustrates, the sensor assembly comprises anaccelerometer with at least one measurement axis and a gyrometer with atleast one measurement axis, the measurement axes being, for example,directly orthogonal to the accelerometer A and to the gyrometer Gcorrespond respectively to a first transverse axis x in the direction ofthe width of the racket Ra, a second transverse axis y in the directionof the thickness of the racket Ra, and a longitudinal axis z orientedfrom the shaft to the head of the racket Ra. In the case of tennis, theassociated projectile Pj is a tennis ball.

In a variant, as illustrated in FIG. 2 a, the axes of the sensorassembly can be different from the axes of the racket (transverse in thedirection of the width of the racket Ra, transverse in the direction ofthe thickness of the racket Ra, and oriented from the shaft toward thehead of the racket Ra). In this case, as illustrated in FIG. 2 a, thesensor assembly can comprise a first axis x_(c) offset by 45° withrespect to a transverse axis x in the direction of the width of theracket Ra and a second axis z_(c) offset by 45° with respect to atransverse axis z in the direction of the length of the racket.

In this embodiment, the risks of sensor saturation can be reduced.Indeed, at the beginning of a service, there is a high accelerationalong the z-axis because the racket Ra is tangent to the trajectory ofthe racket Ra, as illustrated on the right-hand part of FIG. 14. In thisembodiment, the acceleration Az along the z-axis of the racket ismeasured by the sensors along the axes x_(c) and z_(c). Each sensormeasures an acceleration Az/√{square root over (2)}. Consequently, it ispossible to measure a higher acceleration Az along the z-axis (by afactor of √{square root over (2)}) before the accelerometers begin tosaturate.

By analogy, it is possible to deduce that a similar advantage exists inthe computation of the rotational velocity along the first transverseaxis x of the racket with the gyrometer G. During the typical strokes ofa racket, the rotational velocity is highest along the first transverseaxis of the racket.

In the embodiment in FIG. 2 a, the y-axis of the sensors is identical tothe second transverse axis y of the racket Ra. If, furthermore, they-axis of the sensors is turned to 45 degrees with respect to the secondtransverse axis y of the racket Ra, by analogy, a factor of √{squareroot over (3)} is gained instead of a factor of √{square root over (2)}because the measurements are made along three axes instead of along twoaxes.

When an impact is detected, a moment of impact t₀ is associated fromvariations over time with the axial linear accelerations Ax, Ay and Azdelivered by the accelerometer A along the x-, y- and z-axesrespectively.

Furthermore, one eliminates impacts not linked to strokes from a set ofstrokes predetermined from the angular rotational velocities Gx, Gy andGz along the x-, y- and z-axes. Thus, as illustrated in FIG. 3, if theimpact originates from a stroke not forming part of the set of strokes,the analysis is not continued. In a variant, in combination, a modulefor determining the attitude of the racket Ra can be incorporated, asillustrated in FIG. 4.

The set of predetermined strokes can, for example, comprise thefollowing strokes: service, forehand, and backhand.

An impact not linked to a stroke from this set can correspond to animpact of the racket Ra on a sports shoe to dislodge clay, or correspondto rebounds of the ball on the stringbed of the racket Ra made by aplayer who is going to serve, between two periods of play. For example,for such strokes, the stringbed of the racket is horizontal, and it cantherefore not be a service, forehand, or backhand.

For example, it is possible to detect an impact of the projectile Pj onthe racket Ra when a norm of an acceleration variation vector DV A=(DVAx; DV Ay; DV Az), the components of which are the temporal derivatives

$\left( {\frac{{Ax}}{t};\frac{{Ay}}{t};\frac{{Az}}{t}} \right)$

of the axial linear accelerations (Ax; Ay; Az), is above a threshold S1.In a variant, it is possible to detect an impact of the projectile Pj onthe racket Ra when a norm of an acceleration variation vector DV A=(αDVAx; βDV Ay; γDV Az), the components of which are based on the temporalderivatives

$\left( {{\alpha \frac{{Ax}}{t}};{\beta \frac{{Ay}}{t}};{\gamma \frac{{Az}}{t}}} \right)$

of the axial linear accelerations (Ax; Ay; Az), (α, β and γ beingarbitrary) and is above a threshold S1.

The norm can, for example, be the Euclidian norm or norm 2(∥DVA∥₂=√{square root over (|DVAx|²+|DVAy|²+|DVAz|²)}), the norm 1(∥DVA∥₁=|DVAx|+|DVAy|+|DVAz|), the norm p(∥DVA∥_(p)=(|DVAx|^(p)+|DVAy|^(p)+|DVAz|^(p))^(1/p)), or the infinitenorm (∥DVA∥_(∞)=max(|DVAx|; |DVAy|; |DVAz|)).

Indeed, for impact detection, even in the event of saturation of theaccelerometer A, a large variation can be detected by comparison with athreshold, in this case the threshold S1.

For example, an impact is detected by testing if the norm 1 of DVA isabove the threshold S1, for example equal to 11:

${{\frac{{{Ax}(t)}}{t}} + {\frac{{{Ay}(t)}}{t}} + {\frac{{{Az}(t)}}{t}}} > {S\; 1}$

The value of the threshold S1 is conventionally obtained by training ona test basis.

Furthermore, it is possible to take into account, for the detection ofan impact, a first comparison between a first value representing theangular rotational velocity G_(x) along the first transverse axis xduring a first interval of time Δt1 immediately preceding the impact anda threshold S2. False impact detections are avoided, corresponding forexample to a rotation of the racket about the y-axis, which cancorrespond to a striking of the racket against a shoe or the net. Inthis case it is taken into account that the main axis of the movement isthe first transverse axis x.

This first comparison can, for example, be written in the followingform:

∫_(t1) ^(t0) |G _(x)(t)|dt>S2,

wherein ∫_(t1) ^(t0)|G_(x)(t)|dt represents the first value and thethreshold S2 can, for example, have a value of 30, t₀ being the time ofimpact and t₁ preceding the impact, for example t₁=t₀−50 ms. It is alsopossible to choose another interval of time so as to select the signalportions that are appropriate and representative of the movement of theracket around the impact, without experiencing the shock effects thatgenerally distort the signals of the sensors.

The value of the threshold S2 is conventionally obtained by training ona test basis. S2 is low so as not to miss strokes lacking power.

It is also possible to take into account, separately or in combinationwith the previous, for the detection of an impact, a second comparisonbetween a first value representing the angular rotational velocity G_(x)in relation to the first transverse axis x during the first interval oftime Δt1 immediately preceding the impact and a second valuerepresenting the angular rotational velocity G_(y) in relation to thesecond transverse axis y during the first interval of time Δt1. Falseimpact detections, notably lateral impacts, for example when the playerhits the side of his racket Ra against his sports shoes to remove theclay, or else when he hits the side of his racket Ra in his hand, areavoided. Thus one takes into account the fact that the movement alongthe first transverse axis x is greater than along the two other y- andz-axes.

This second comparison can, for example, be written in the followingform:

∫_(t₁)^(t₀)G_(x)(t) t > C 1 × ∫_(t₁)^(t₀)G_(y)(t) t

wherein:∫_(t1) ^(t0)|G_(x)(t)|dt represents the first value,C1×∫_(t1) ^(t0)|G_(x)(t)|dt represents the second value, andC1 is a criterion that can for example have a value of ½.

Furthermore, it is possible to determine the attitude or the orientationof the racket Ra with respect to a terrestrial frame of reference fromthe axial linear accelerations Ax, Ay and Az and from the angularrotational velocities Gx, Gy and Gz along the x-, y- and z-axes, asillustrated in FIG. 4.

Several physical devices and algorithm types can be used to estimate theattitude, the angular velocity of the attitude of an object equippedwith sensors of accelerometer and gyrometer type.

Concerning modules for computing orientation using these sensor types,reference is made to the products of Movea™ or the products of XSens™,or Intersense™ with, for example, the product family InertiaCube™ or theproducts of CrossBow™.

Reference is made, for example, to the article “An extended Kalmanfilter for quaternion-based orientation estimation using MARG sensors”by Marins, J. L., Xiaoping Yun, Bachmann, E. R., McGhee, R. B., andZyda, M. J., published in “Intelligent Robots and Systems”, 2001, 2001IEEE/RSJ. This article gives access to many other references that it isuseful to analyze.

Concerning gyrometers, it is, for example, possible to use thegyrometers supplied by Analog Devices™ with the reference ADXRS300™, orthe ITG3200™ from Invensense™ or the gyrometers supplied by STM™.

Concerning accelerometers, it is, for example, possible to use theaccelerometers with the reference ADXL103 from Analog Devices™ andLIS302DL by STM™. FreeScale™ and Kionix™ also supply such sensors.

Various algorithms can be used to correct the perturbations and/ordefault of each sensor and thus merge the signals of the various sensorsand therefore estimate the attitude. Reference is made, for example, toInternational patent application no. WO2010/007160, the contents ofwhich are incorporated herein by reference. Alternative methods may alsobe used. The best-known algorithms are the Kalman filter, optimizationmethods, or additional filtering methods.

Qualitatively, to supply the orientation data item, it is possible touse inertial devices embedded in the object comprising accelerometer andgyrometer combinations. The accelerometers make it possible to measurethe orientation of the object with respect to a fixed vector related tothe earth, i.e. terrestrial gravity. The gyrometers measure the inherentangular velocity of the movements of the object. The gyrometers aregenerally affected by a significant temporal drift that must beregularly corrected. The accelerometer makes it possible to supply anabsolute orientation with respect to a terrestrial frame of reference.Gyrometers are effective for estimating orientations during phases ofrapid movements, between two absolute orientations.

The sensors can be microelectromechanical systems or MEMS, optionallyintegrated, or made using other non-integrated technologies. Each typeof sensor can include one, two or three axes. Today, it is natural tointegrate sensors with three axes in products, this technology now beingcommonplace. In some applications, a single sensor type (in this casegenerally with three axes) can be used, if the perturbations or temporaldrift can be considered negligible so that the final orientation dataitem, desirable for embodiments of the invention, is precise enough, orbe corrected without resorting to another sensor type. Ideally, however,a combination of at least two sensor types will be used for embodimentsof the invention, for example accelerometer and gyrometer.

Note that these two sensor types can contribute complementaryinformation with a view to estimating the orientation. The tri-axialversion gyrometer supplies angular velocity measurements in relation tothree Degrees of Freedom (DOF), and makes it possible to estimate theattitude by integration of the angular velocity. It therefore makes itpossible to compute a relative orientation with respect to a givenorientation. This principle of estimating the orientation is subject toa drift because of the integration operation and the gyrometer bias, ifthe gyrometer is used alone. The tri-axial version accelerometersupplies two items of angular information (the angles of roll and yaw)that are absolute with respect to a terrestrial frame of reference, butis subject to perturbations when the movements are not quasi-staticsince it measures at the same time the acceleration parameters due tothe movement. The combination of the two sensors makes it possible tosupply measurements of absolute attitude with respect to a terrestrialframe of reference, with the exception of the heading (angle withrespect to the North in the terrestrial frame of reference) with respectto the earth, whose value can only be estimated using the gyrometer andtherefore as a relative value with respect to a reference orientation.

Given the attitude of the racket Ra, a stroke is detected among a set ofstrokes, by associating a stroke with a movement for which, at themoment of impact, the velocity of the racket Ra is essentially along adetermined axis, as illustrated in FIG. 5. For example, for a service,using the projection of the z-axis onto the gravity g, and the way theuser lifts his racket is tracked.

Furthermore, it is also desirable to use, for detecting a stroke among aset of strokes, an item of information representing the left- orright-handedness of the player, and representing the direction of themovement determined from the sign of the angular rotational velocityalong the determined axis. The information item representing the left-or right-handedness of the player can be supplied by the player orlearnt during the game. During a service, the player turns the racketaround the z-axis, and the direction of the movement gives theinformation on the left- or right-handedness of the player (RotationGz>0: right-handed, and Gz<0: left-handed).

Furthermore, it is also possible to use for the detection of a strokeamong a set of strokes, an associating a stroke with a determined formof the projection of the attitude of the racket Ra onto a determinedaxis over an interval of time comprising at least one portionimmediately preceding the impact.

To illustrate what has just been said, the following set is taken as theset of strokes: {service; forehand; backhand}. The axes of interest arethe z-axis for a service and the x-axis for a forehand or a backhand, asillustrated in the following example.

To determine whether a stroke is a service, a tracking of theorientation of the longitudinal axis {right arrow over (z)} of theracket Ra with respect to the gravity vector {right arrow over (g)} isused, as illustrated in FIG. 6. The attitude of the racket Ra isdetermined from the measurements from the accelerometer A and thegyrometer G, then the projection of the longitudinal axis {right arrowover (z)} of the racket Ra on the gravity vector {right arrow over (g)}is computed. As illustrated in FIG. 6, according to the position of theracket Ra, the value of the projection of {right arrow over (z)} onto{right arrow over (g)} can be seen.

Thus, when the racket Ra is vertical oriented upwards, this projectionhas a value of −1, when the racket Ra is horizontal, this projection hasa value of 0, and when the racket Ra is vertical oriented downward, thisprojection has a value of 1.

FIG. 7 illustrates an example of the evolution of this projection over aservice, during which, at the moment of impact of the ball (impactdetection peak PDA), the racket Ra is substantially vertical orientedupward, and the projection of {right arrow over (z)} onto {right arrowover (g)} has a value of substantially −1.

Different players can have different services. To cover all types ofservice, different criteria can be used, for example three criteria areused to determine a service:

-   -   the first criterion C1: the projection of the projection of        {right arrow over (z)} onto {right arrow over (g)} at the moment        of impact must correspond to a substantially vertical position        oriented upward, i.e. C1<−0.82. The majority of services are        detected by this criterion.    -   a second criterion C2: the amplitude of the movement,        represented by the difference between the projection of {right        arrow over (z)} onto {right arrow over (g)} at the local maximum        P2 before the impact and the projection at the local minimum P3        at the moment of the impact, i.e. C2>1.71. Before testing the        criterion C2, it is made sure that at the moment of impact the        projection of {right arrow over (z)} onto {right arrow over (g)}        is below −0.5. This criterion makes it possible to detect a        service for which the ball is struck before or after the        vertical position oriented upward of the racket Ra.    -   C3: the projection at the local maximum P2 C3>0.84. Before        testing the second criterion C2, it is desirable to be sure that        at the moment of impact the projection is below −0.5. This        criterion covers a service during which the player projects the        racket Ra quite far to the back, but strikes the ball with the        racket Ra not totally vertical.

Generally, the thresholds required for the operation of the method canbe advantageously determined on a test basis representing the scenariosof use.

These criteria are tested in the order C1, C2, C3, and if one of them ismet, a service is detected, If none of the three criteria is met, noservice is detected. Note that the second criterion C2 and the thirdcriterion C3 comprise time constraints.

Next or in parallel, it is possible to detect if the stroke performed bythe player is a forehand or backhand stroke.

The service movement can also be used to determine whether the player isleft- or right-handed. During the swing movement in the service theplayer turns the racket Ra about the x-axis. This rotation can bemeasured by the gyroscope z, and from its sign, it is possible to deduceif the player is left- or right-handed (Rotation Gz>0: right-handed, andGz<0: left-handed).

To tell a forehand stroke apart from a backhand, three factors arepreferably taken into account.

-   -   first of all, the dominant hand: it is desirable to determine        whether the player is left- or right-handed. It is desirable to        tell between a forehand stroke of a right-handed player and a        backhand stroke of a left-handed player, as well as between a        backhand stroke of a right-handed player and a forehand stroke        of a left-handed player. The dominant hand can be deduced from        the swing of the service, or can be known from an input by the        user;    -   next the orientation of the racket Ra: to determine the        orientation of the racket Ra, the orientation of the axis {right        arrow over (x)} of the racket Ra with respect to the gravity        vector {right arrow over (g)} is computed. The sign of the        projection of {right arrow over (x)} onto {right arrow over (g)}        gives us the orientation of the racket Ra as illustrated in FIG.        8, the x-axis is vertical oriented downward when the projection        of {right arrow over (x)} onto {right arrow over (g)} has a        value of 1 and the x-axis is vertical oriented upward when the        projection of {right arrow over (x)} onto {right arrow over (g)}        has a value of −1;    -   finally, the direction of the movement before the impact: the        direction of the movement is determined from the sign of the        angular rotational velocity supplied by the gyrometer G relative        to the axis {right arrow over (x)}.

If the value 1 is associated with a right-handed player and the value −1with a left-handed player, it is possible to determine the type ofstroke by multiplying these three factors. If the result is positive(i.e. has a value of 1) the stroke is a forehand stroke, and if theresult is negative (i.e. has a value of −1) the stroke is a backhandstroke. The various cases are represented in FIG. 9.

It is also possible, in a variant or in combination, to determine thepresence of an effect given to the projectile Pj at the moment of impactfrom a third comparison of the angular rotational velocity along an axiswith the angular rotational velocity along another axis, for example ofthe angular rotational velocity G_(x) along the first transverse axis xduring a first interval of time immediately preceding the impact and ofthe angular rotational velocity G_(y) along the second transverse axis yduring said first interval of time.

A backspin effect is distinguished from a topspin effect from the signof the angular rotational velocity G_(y) along the second transverseaxis y and of the orientation or attitude of the racket Ra during saidfirst interval of time.

For example, the player can give a backspin or topspin effect to theball by adjusting the angular velocity Gy of the racket Ra upon impact.A stroke with no effect has an angular rotational velocity only in thex-direction, represented by a gyrometric signal along Gx. An effect isgiven by applying a rotation about the y-axis, thereby increasing thegyroscopic signal along Gy. A stroke is considered to have an effectwhen |Gy|/(|Gx|+|Gy|)>S3, S3 being a threshold for example equal to 0.4.

The value of |Gy|/(|Gx|+|Gy|) can also be used to have a parametercorresponding to the intensity of the effect.

The type of effect, backspin or topspin, depends on the sign of Gy andthe orientation of the X-axis of the racket with respect to the gravityvector:

(sign (Gy))*(projection of {right arrow over (x)} onto {right arrow over(g)})>0 backspin effect

(sign (Gy))*(projection of {right arrow over (x)} onto {right arrow over(g)})<0 topspin effect

In a variant or in combination, as illustrated in FIG. 10 it is possibleto determine a longitudinal impact strip, wherein the impact of theprojectile Pj has taken place, from a variation of the angularrotational velocity G_(z) along the longitudinal axis z over an intervalof time immediately following the impact, corrected by an item ofinformation representing the velocity of the racket Ra just beforeimpact.

In the example of FIG. 10, several longitudinal impact strips aredefined, in this case three longitudinal impact strips, a high-impactstrip, a medium-impact strip, and a low-impact strip.

In the event of an impact outside the z-axis, the gyrometer Gz thatmeasures the angular rotational velocity Gz about the z-axis displays anoscillation, as represented in FIG. 11. The downward slope representsthe rotation of the racket Ra by reason of the impact, and there-ascending slope is due to the overcompensation by the action of theplayer's wrist.

The amplitude of the oscillation max(Gz)−min(Gz) is taken as ameasurement of the rotation effect due to an impact outside the axes.The problem with this measurement is that a high impact velocityslightly outside the axes, and a low-velocity impact near the edge ofthe racket Ra, have the same effect. This means that it is desirable tocompensate for the velocity, i.e. the power of the stroke E_(GXY) justbefore impact. This energy can be represented by the followingrelationship:

E _(GXY) =|G _(X) |+|G _(Y)|

Wherein Gx and Gy represent the angular rotational velocities ofrotation about the x- and y-axes. A normalization using {right arrowover (E_(GXY))} works well. To determine if an impact is inside themiddle strip or further outside the axis, we introduce C defined by thefollowing relationship:

$C = \frac{{\max \left( G_{Z} \right)} - {\min \left( G_{Z} \right)}}{\sqrt{E_{GXY}}\left( {t_{0} - {0.1\; s}} \right)}$

A threshold C for example equal to 2 can be used to tell the differencebetween impacts in the medium band (C<2), and impacts outside the axes(C>2). The threshold can be kept fixed, or can be slightly modified(between 1.7 and 2.1) according to the type of stroke (service,backhand, forehand stroke etc.) in order to increase precision. The bestthreshold is determined by training on a test basis.

By observing the sign of the drift dGz/dt at the start of theoscillation, we can determine on which side of the center of the impacthas taken place.

In a variant or in combination, an impact of the projectile Pj on theracket Ra can furthermore be detected from vibration measurementstransmitted by a piezoelectric sensor mounted in a fixed manner on theracket Ra, by comparing a parameter representing vibrations with athreshold. It is for example possible to integrate the signal over afrequency window and to compare the result with this threshold.

For example, as illustrated in FIG. 12, one determines, among severalradial impact strips, in this case three radial impact strips B1, B2,and B3, that wherein the impact of the projectile Pj has taken place,from the energy and the phase of the signal transmitted by thepiezoelectric sensor P just after impact.

When a ball strikes the racket Ra, it creates vibrations, and thesevibrations depend on the position of the impact. By positioning apiezoelectric sensor P between the grip and the bottom of the stringbedof the racket Ra, it is possible to measure the vibrations in such a wayas to deduce the position of the impact.

To compute the position of the impact, it is desirable to analyze thefundamental vibration peak in the frequency spectrum (around a frequencythat can depend on the racket Ra, in the present example, f_(n)=160 Hz)and to determine the energy E_(E1) and the phase Φ₁ of the signal.

The following relationships exist:

E _(E1) =∫s _(mn) ²(t)dt, and

$\Phi_{1} = {{{atan}\left( \frac{{Im}\left( {s_{mn}\left( f_{n} \right)} \right)}{{Re}\left( {s_{mn}\left( f_{n} \right)} \right)} \right)} = {{atan}\left( \frac{\int{s_{mn}\sin \; 2\; \pi \; f_{n}t{t}}}{\int{s_{mn}\cos \; 2\; \pi \; f_{n}t{t}}} \right)}}$

wherein:Im represents the “imaginary part” function,Re represents the “real part” function, andS_(mn)(t) is a time-frequency selection of the signal s(t) provided bythe piezoelectric sensor P, for example s(t) for the first 35 ms afterimpact and for frequencies between 50 Hz and 300 Hz around the centralpeak (f_(n)=160 Hz).

In the case of a separation of the stringbed into radial strips, thesignal s(t) conveyed by the piezoelectric sensor P makes it possible todetermine in which radial strip the impact has occurred.

FIG. 13, which represents the phase Φ₁ of the signal s(t) as a functionof the normalized energy E_(E1), does indeed show that it is possible todeduce therefrom a radial strip to which the impact position belongs, inthis case B1, B2 or B3.

It should be noted that these features may depend on the type of racketRa, which means that the procedure can be refined for each racket Ra.For certain rackets it can be beneficial to use two different frequencybands and to use the ratio of the respective normalized energies ofthese two frequency bands for the x-axis.

By combining the determinations of longitudinal and radial impact zone,an impact zone along the axes x and z of the racket Ra is determined, ina precise manner.

Furthermore, independently or in combination, it is possible, whenstarting a phase of play, for example during a service, to compute alaunch velocity of the projectile Pj on the basis of the axialacceleration values during the acceleration phase of the racket Rapreceding the impact and in the direction of the stroke starting thephase of play.

For example, it is possible to correct the velocity computed from theknowledge of a zone of impact of the projectile Pj and/or of thepresence of an effect.

It is possible to compute the ball velocity during a service because itis possible to suppose that the ball has no initial velocity or in otherwords has zero initial velocity: V(t0)=0.

If it is supposed that at the start of the swing or during the swingingmovement of the service, the racket Ra has zero velocity, it is possibleto compute the velocity of the racket Ra just before the impact usingthe measurements of the accelerometer and/or gyrometer of the racket Ra.Furthermore, it is supposed that the ball velocity after the impact isequal to the velocity of the racket Ra just before the impact. Indeed,it is supposed that at the moment of the impact, the racket Ra and theprojectile Pj form a single system, and that the projectile Pj thustakes the velocity of the racket Ra.

In practice, it is frequently not possible to measure the accelerationand the angular rotational velocity, just before impact, because thesensors can be saturated by the impact. Consequently, the accelerationof the racket Ra at the start of the swing is measured. Furthermore, atthe start of the swing, the longitudinal axis z of the racket Ra istangent to the trajectory, which means that the angular rotationalvelocity can be neglected, as illustrated in FIG. 14.

Indeed, FIG. 14 represents the evolution over time of the position ofthe racket Ra during a service, for various intermediate positions ofthe racket Ra. In FIG. 14, ω_(pod) represents the angular velocity ofthe racket measured by the gyrometer or gyrometers, θ′ represents theangular velocity linked to the trajectory or the velocity of the hand ofthe player, and R represents the instantaneous radius of curvature ofthe trajectory.

At the start of the service swing, the racket Ra makes a “pause” behindthe back of the player. This pause can be considered as a local minimumof the acceleration, as illustrated in FIG. 15. This minimum can beconsidered as the start of the movement. However, for players having ahigh standard of play, this minimum may not exist, or not correspond tothe start of the movement.

Several criteria are taken into account at the place of the accelerationminimum:

-   -   the energy on impact, translated by the value of the        acceleration (sum of the acceleration components along the x-,        y- and z-axes) just before t₀.    -   the slope of the progression of the energy over a few data        samples before the impact.

All this can take into account the fact that at least one of thecomponents of the acceleration saturates before impact, over a timeframe that is more or less wide according to the level of the player.This time frame can extend over 5 or 6 samples for beginner players(either for one sampling every 5 ms, over 25 or 30 ms), up to 30 samplesfor the most energetic (150 ms).

The energy on impact can therefore essentially rely on the values of theother components of the acceleration. The higher the energy, the longerthe time period before impact over which to integrate.

As to the slope of the acceleration profile, it is the mostrepresentative over the 50 ms before impact. The smaller the slope, thegreater the acceleration before the impact, and the wider the chosenrange of integration to obtain the velocity. If the movement is verystrong, the acceleration along the x-axis saturates very quickly and thecorresponding signal is constant (horizontal). Because of this, theslopes of the profile of the accelerations computed from the three axesare small.

In summary, the following pattern exists:

MaxAcc representing the acceleration maximum, over all the axes, reachedjust before impact, and Acc50 representing the acceleration to (t₀−50ms).If MaxAcc is below 35, the integration is carried out over 10 samplesbefore impact.If MaxAcc is between 35 and 45, the integration is carried out over 15samples before impact.If MaxAcc is above 45, the integration is carried out over 18 samplesbefore impact.

For each of the three previous tests, the following test is added:

If (MaxAcc−Acc50) is above 10.5, the integration is carried out overfour more samples (which gives 14, 19, 22 respectively according to thevalue of MaxAcc).

The estimated velocity can be corrected, taking into account thepresence of an effect. In the case of the service, the effect can be atopspin or a slice, for example.

Effects are accounted for in the following manner: at the moment ofimpact the value of

$\alpha = \frac{G_{Y}}{{G_{X}} + {G_{Y}}}$

is measured: the greater the rotation about the y-axis, the more markedthe effect.

This parameter α is between 0 and 1. The corrected velocity v′ iscomputed by the relationship v′=v×(0.2−α⁴); indeed, the greater theeffect, the more the velocity of the ball or projectile Pj is sloweddown with respect to the velocity of the racket. A function decreasingwith α makes it possible to re-evaluate the velocity.

Furthermore the velocity can, independently or in combination, becorrected taking into account the centering of the ball, by using thecentering criterion C previously defined.

The corrected velocity v″ taking the centering into account follows thefollowing relationship:

v″=v(1+sign(E(C−0.8))×(1.2843−0.1857×C))

with E representing the integer part function, giving:if C<1.8 sign (E(C−0.8))=0

=else 1

If this correction is applied in combination with the correction takingan effect into account, i.e. after having taken the effects into accountusing the parameter α described earlier, the centering is also takeninto account: indeed, a service wherein the ball is off-center will alsobe overestimated. The corrected velocity v″ taking the centering intoaccount follows the following relationship:

v″=v′(1+sign(E(C−0.8))×(1.2843−0.1857×C))=v×(0.2−α⁴)×(1+sign(E(C−0.8))×(1.2843−0.1857×C))

E being the integer part function, giving:if C<1.8 sign (E(C−0.8))=0

=else 1

The numerical values, such as 1.8, taken into account were obtained bytests.

There will therefore be no change in the velocity if the centeringcriterion is beneath the threshold of 1.8 (1.8 is chosen according tothe last centering results).

Generally, the thresholds required for the operation of the method canadvantageously be determined on a test basis representing the scenariosof use.

As illustrated in FIG. 16, the velocity of the projectile is computed inkm/h, using training by means of a comparison with radar measurements oroptical devices (here a Vicon™ device). The scatter of points obtained,by tests of different players, is represented in FIG. 16. Also, in thiscase, a linear relationship is identified between the velocity estimatedby the algorithm or a quantity representing the velocity estimated bythe algorithm, and the velocity measured by radar, which makes itpossible to adapt and correct the computed velocity. This method can beeasily generalized to other relationships, using polynomials of order 2,3 or another parameterized function, etc., in order to reproduce morecomplex functions than a linear relationship. Correspondence tables canalso be used, or any other method of function approximation (for exampleneural networks.)

The location of the player in the game space can be added from dataprovided by a location system receiver, for example a satellite locationsystem, linked in displacement to the player or to the racket, or asystem comprising a video camera.

When the velocity of the racket exceeds a limit rotational velocity ofthe gyrometer, an extrapolation can be carried out, or a hypotheticalextension of a law, of a function or of a quantity beyond the timelimits wherein they are objectively observed, of signals provided by thegyrometer over this saturation period. Indeed, a saturated sensor isless accurate, so using an extrapolation, or a hypothetical extension ofa law, of a function or of a quantity beyond the time limits whereinthey are objectively observed makes it possible to improve accuracy.

Embodiments of the present method makes it possible to provide, in realor delayed time, qualitative and/or quantitative statistics relating tothe player's way of playing, by way of a terminal screen, for example atouch-sensitive tablet.

The core data are computed in a computer embedded in the racket, so thatin the event of a problem of transmission of the data from the racket tothe mobile terminal equipped with display means, the data are notcorrupted.

1. A method for analyzing a game of a user of a racket, wherein: animpact on the racket is detected from representative measurements of ashock to the racket provided by a sensor assembly comprising at leastone sensor sensitive to shocks coupled to the racket in a fixed mannerin terms of movement; a moment of impact is associated with a detectedimpact, from the measurements transmitted by the sensor assembly; andimpacts that are not related to strokes from a set of pre-determinedstrokes are eliminated utilizing angular rotational velocitymeasurements, provided by a gyrometer of the sensor assembly, thegyrometer having at least one measurement axis, the angular rotationalvelocity measurements taken during an interval of time around saidmoment of impact.
 2. The method as claimed in claim 1, wherein saidsensor assembly comprises at least one vibration sensor, and an impacton the racket is detected utilizing measurements of vibrationstransmitted by the vibration sensor, by comparison of a parameterrepresenting the vibrations with a threshold.
 3. The method as claimedin claim 1, wherein said sensor assembly comprises an accelerometer withat least one measurement axis, and an impact on the racket is detectedwhen a parameter depending on the variations over time of axial linearaccelerations or variations over time of the angular rotationalvelocities is above a threshold.
 4. The method as claimed in claim 1,wherein impacts that are not related to strokes from a set ofpredetermined strokes are eliminated utilizing a comparison between avalue representing the angular rotational velocity along an axis duringan interval of time around the moment of impact and a threshold.
 5. Themethod as claimed in claim 1, wherein said gyrometer at least twomeasurement axes, impacts that are not related to strokes from a set ofpredetermined strokes are eliminated utilizing a comparison between afirst value representing the angular rotational velocity along a firstaxis during an interval of time around the moment of impact and a secondvalue representing the angular rotational velocity along a second axisduring said interval of time.
 6. The method as claimed in claim 1,wherein said gyrometer comprises three measurement axes and said sensorassembly further comprises at least one accelerometer with threemeasurement axes, an attitude of the racket is determined with respectto a terrestrial frame of reference from the measurements of axiallinear accelerations and the measurements of the angular rotationalvelocities along said measurement axes.
 7. The method as claimed inclaim 6, wherein a stroke is classified among a set of strokes byassociation of a stroke with a movement of the racket for which, aroundthe moment of impact, the rotational velocity of the racket issubstantially along a determined axis and the attitude of the racket issubstantially at a determined attitude.
 8. The method as claimed inclaim 7, wherein a stroke is further classified among a set of strokesutilizing an item of information representing left- or right-handednessof the player, and of a sign of the angular rotational velocity alongthe determined axis.
 9. The method as claimed in claim 8, wherein theitem of information representing the left- or right-handedness of theplayer is provided by the player or determined automatically during thegame.
 10. The method as claimed in claim 7, wherein a stroke is detectedamong a set of strokes by associating a stroke with a determined form ofa projection of a vector representing the attitude of the racket onto anaxis determined over an interval of time around the moment of impact.11. The method as claimed in claim 1, wherein said gyrometer comprisesat least two measurement axes and the impact is due to a projectile, anintensity of an effect given to the projectile is determined, at themoment of impact, from a comparison of the angular rotational velocityalong a first axis during an interval of time around the moment ofimpact and of the angular rotational velocity along a second axis duringsaid interval of time.
 12. The method as claimed in claim 11, whereinsaid axes comprise a first transverse axis in a direction of a width ofthe racket and a second transverse axis in a direction of a thickness ofthe racket, and a backspin effect is differentiated from a topspineffect utilizing a sign of the angular rotational velocity along thesecond transverse axis and an orientation of the racket during saidinterval of time.
 13. The method as claimed in claim 12, wherein theaxes further comprise a longitudinal axis oriented from a shaft to ahead of the racket, said gyrometer with at least one measurement axisbeing configured to provide a rotational velocity along the longitudinalaxis, and the impact being due to a projectile, a longitudinal impactstrip on a stringbed of the racket, wherein the impact of the projectileis determined from a variation of the angular rotational velocity alongthe longitudinal axis over an interval of time immediately following theimpact.
 14. The method as claimed in claim 13, wherein said gyrometerwith at least one measurement axis is configured to provide a rotationalvelocity along the first transverse axis in the direction of the widthof the racket, said determination of the longitudinal impact strip iscorrected by an item of information representing the velocity of theracket around the moment of impact.
 15. The method as claimed in claim1, wherein the sensor assembly comprises at least one vibration sensorand the impact is due to a projectile with a radial impact strip (B1,B2, B3), wherein the impact of the projectile is determined utilizing anenergy and a phase of a signal transmitted by the vibration sensor dueto the impact.
 16. The method as claimed in claim 1, wherein the sensorassembly comprises an accelerometer with three measurement axes or agyrometer with three measurement axes, the impact being due to aprojectile, and during a start of a phase of the game, a launch velocityof the projectile is respectively computed from measurements of axialaccelerations or measurements of angular rotational velocities during aphase of acceleration of the racket preceding the impact.
 17. The methodas claimed in claim 16, wherein the computed launch velocity of theprojectile is corrected utilizing knowledge of a zone of impact of theprojectile or of an intensity of an effect given to the projectile. 18.The method as claimed in claim 1, wherein a location of the player on agame space is determined from data provided by a system for locating theplayer or the racket.
 19. The method as claimed in claim 1, wherein thesensor assembly comprises at least one accelerometer, and when during aninterval of time around the moment of impact the signals of said sensoror sensors are saturated, an extrapolation of the signals provided bythe sensor or sensors is carried out over said saturation interval oftime.
 20. A system for analyzing a game of a user of a racket,comprising: detecting means for detecting an impact on the racket frommeasurements representing a shock experienced by the racket providedfrom a sensor assembly comprising at least one sensor sensitive toshocks coupled to the racket, the detecting means comprising a gyrometerwith at least one measurement axis; associating means for associating amoment of impact with a detected impact, utilizing the measurementstransmitted by the sensor assembly; and eliminating means foreliminating impacts not related to strokes from a set of predeterminedstrokes utilizing angular rotational velocity measurements, provided bysaid gyrometer, taken during an interval of time around said moment ofimpact.
 21. The system as claimed in claim 20, wherein said sensorassembly is mounted on the racket in a fixed manner in such a way thattwo measurement axes of said sensor assembly form an angle of 45° with afirst transverse axis in a direction of a width of the racket and alongitudinal axis in a direction of a length of the racket.
 22. Thesystem as claimed in claim 20, wherein said sensor assembly is mountedin a fixed manner in an outer casing equipped with fixing means adaptedfor being mounted or dismounted as desired on the racket, or is mountedon the racket in a fixed manner.
 23. The system as claimed in claim 20,wherein said sensor assembly is mounted in a fixed manner on the racketand comprises an accelerometer and a gyrometer arranged in a shaft ofthe racket at a bottom of a grip, and a vibration sensor is arranged onthe shaft of the racket between the grip and a bottom of a head of theracket.